This invention relates generally to the field of wellbore completions in oil and gas production operations and more specifically to the design of gravel pack completions that are better able to withstand reservoir compaction forces.
The production of fluids from subterranean reservoir formations can lead to a reduction in reservoir pressure. In some cases, the pressure reduction results in crushing of the reservoir rock by the weight of overlying formations. This crushing of the reservoir rock and the related subsidence is sometimes referred to as reservoir compaction. The movement of subterranean formations associated with reservoir compaction can damage wellbores that intersect the deforming or moving formations. Reservoir compaction may also manifest itself as a collapse around a horizontal wellbore, which generally also damages or destroys the wellbore.
One method currently used to protect a well completion from formation movement is the creation of an annular space (or spaces) between the wellbore tubing and the surrounding subterranean formation and/or casing, to eliminate or postpone the transfer of mechanical forces (load) from a moving formation to the completion tubulars and any associated downhole equipment. Within a production interval, a fluid-filled annular space could exist in either an open-hole completion or in a cased-hole completion. The annular space allows for some formation movement to occur before force is applied to the well completion equipment. Note that in a cased-hole completion, the load is first transferred to the outer casing and then to the wellbore tubing string as the outer casing is deformed. One variation of this method is to enlarge the initial borehole with bi-center drill bits or underreaming (hole opening) operations. The enlarged borehole provides additional space for formation movement before loads are applied to the wellbore components.
In some reservoirs, particulate matter is carried into the wellbore along with the produced fluids. Such particulate matter, hereafter referred to as xe2x80x9csand,xe2x80x9d may result from an unconsolidated or loosely consolidated formation, from a consolidated formation with intervals of friable or unconsolidated material, or from crushing of the formation due to compaction. In such a situation, a xe2x80x9cgravel packxe2x80x9d or other filtration method may be employed to avoid damaging or plugging the wellbore or production equipment with sand. In a typical gravel pack completion, a screen is positioned within the wellbore adjacent to the interval to be completed and a gravel slurry is pumped down the well and into the annulus around the screen. As liquid is lost from the slurry into the formation, gravel is deposited around the screen to form a permeable mass around the screen. The gravel is sized to allow produced fluids to flow through the gravel, and to block the flow of formation particulate material.
Since a gravel pack occupies the annular space between the completion hardware and the formation, using a fluid-filled annular space to protect the wellbore is not possible when a gravel pack is employed to prevent production of formation sand. The gravel pack will occupy the annulus between the casing and screen, allowing transmission of forces directly from the casing to the completion hardware. An enlarged borehole is also not practical when a gravel pack is being used because this borehole-by-casing annulus often must be filed with cement to allow hydraulic isolation of the producing formations or mechanical fixation of the casing. This cement provides a vehicle for transmission of load from the formation to the casing. Completion hardware includes the production tubing string, the screen used to exclude the gravel pack, and any other associated downhole equipment in the affected portion of the wellbore. The screen used to contain the gravel pack in the annulus is particularly vulnerable to damage when the gravel pack is deformed by the force of the moving formation.
Gravel pack gravel can be naturally occurring or man-made, and is typically made up of hard silica-like materials that have compressive strengths similar to or exceeding the compressive strength of the reservoir formation. The term xe2x80x9cconventional gravelxe2x80x9d will be used herein to indicate silica, silica-like, or ceramic particles used as a downhole filtering mechanism. The presence of the conventional gravel enables direct application of forces from the moving or collapsing formation to the wellbore completion equipment for both open-hole and cased-hole gravel pack completions. When a conventional gravel-packed completion is employed to control production of formation sand, the compacting or subsiding reservoir imparts mechanical loads (via the gravel pack gravel) to the well completion equipment, damaging that equipment and often requiring sidetracking around the damaged zone and re-drilling through the desired formation or even drilling of a replacement well.
Reservoir compaction forces transmitted through gravel packs significantly reduce the life of the completion, resulting in significant costs to recomplete or re-drill a well. Postponing, reducing, or eliminating the damage resulting from reservoir compaction would have significant economic benefits. Accordingly, there is a need for a wellbore completion methodology that will allow the wellbore to be flexible and resilient to the movement or collapse of the surrounding formation while controlling the production of formation sand.
This invention provides a method for forming a gravel pack in a wellbore penetrating a subterranean formation by using a solid deformable material in place of at least a portion of the conventional gravel to reduce the transfer of load forces from the subterranean formation to the wellbore equipment below the transfer of load forces that would occur with conventional gravel alone. The invention also provides a gravel pack, which includes a mixture of conventional gravel and a deformable material to form the gravel pack in the wellbore.